1. Field of the Invention
This invention relates to synthetic esters of alcohols and fatty acid mixtures containing at least 85% by weight of oleic acid and 0.5 to 2.5% by weight of stearic acid and to their use as lubricants and hydraulic oils and for cosmetic purposes.
2. Discussion of the Related Art
With the object of improving the yield of renewable raw materials, various oil plants have recently been modified in the composition of their fatty acid mixtures by breeding measures in order to make them more suitable for industrial purposes. Examples include rape, of which the erucic acid content has been lowered, rape with an increased erucic acid content and sunflowers of which the oils have an increased oleic acid content through reduction of their linoleic acid content.
The selective modification of oil plants in order to increase their oleic acid content by reducing their linoleic acid content is described in U.S. Pat. No. 4,627,192. The composition of "high-oleic sunflower oil" is reported, for example, in J. Amer. Oil Chem. Soc. 63, 1062 (1986). The oleic acid content is of the order of 80 to 86% while the content of linoleic acid is between 4 and 8% and the stearic acid content between 3 and 5%.
The fats and oils obtained from vegetable or animal sources are processed to oleochemical raw materials. On account of the C-chain distribution of the fatty acids which is always present in natural fats and oils, mixtures containing fatty acids of a homologous series are obtained instead of pure substances.
Natural triglycerides show such poor stability to hydrolysis or oxidative degradation that they cannot be used as lubricants for many purposes. Synthetic esters of the fatty acid mixtures obtained from them by splitting show superior performance properties.
An important requirement for an ester-based lubricant is a low pour point according to DIN ISO 3016. Esters of trimethylol propane with "oleic acid" represent one possibility of obtaining lubricants of medium viscosity, low cloud point and low pour point. On account of the C chain distribution already mentioned, "oleic acid" is not a pure substance, but a mixture of various fatty acids.
Technical oleic acid is obtained from tallow. To obtain a sufficiently low cloud point, as measured by the method of Deutsche Gesellschaft fur Fettforschung D-III 3 [79], special measures have to be taken to purify the fatty acid mixture, i.e. most of the saturated fatty acids are removed by so-called rolling-up separation. The cloud point of the fatty acid mixture is thus reduced to values of 5.degree. C. to 10.degree. C. The mixture consisting, for example, of 67% oleic acid, 12% linoleic acid and 2% stearic acid is marketed under the name of Edenor.RTM. TiO5 and, on esterification with trimethylol propane, gives esters with a cloud point of -20.degree. C. and a pour point of -40.degree. C.
Unfortunately, the esters produced by known methods as described above are not entirely satisfactory in regard to their oxidation stability on account of their content of oxidation-sensitive linoleic acid. Their color and odor are also in need of improvement.
Other problems arise through quality fluctuations because animal fats varying widely in origin are used as the raw material source. Experience has shown that this can lead to unpredictable difficulties in maintaining the cloud point and pour point specification.
To avoid these problems, it seems logical to use a raw material with a higher oleic acid content, i.e. the high-oleic sunflower oil described at the beginning.
Unfortunately, the use of a fatty acid mixture of sunflower oil from new plants (oleic acid content around 85%, linoleic acid content around 5%, stearic acid content around 4%, palmitic acid content around 4%) does not produce the required result: the cloud point of the trimethylol propane ester is -10.degree. C. while its pour point is -20.degree. C.
However, the rolling-up process remains confined to fatty acid mixtures which still have a relatively high linoleic acid content. Accordingly, the cloud point of a high-oleic fatty acid mixture from new sunflowers cannot be reduced to 5.degree. C. by rolling-up separation because pure oleic acid already has a melting point of 14.degree. C.
Hydraulic oils generally contain no water. Nevertheless, water can enter hydraulic oil through defects or during long-term operation. This water must not be emulsified in the hydraulic oil, but instead should separate quickly as a second phase. This so-called demulsifying capacity of hydraulic oils is measured in accordance with DIN 51599. The demulsifying capacity of esters based on technical oleic acid is in need of improvement.